First application of ranging-out method and hybrid 3Hen counter at ISAC-1: measurement of absolute beta-delayed neutron rates (I βn ) for Ga and Ge isotopes around N=50 collaboration of ORNL Physics - TRIUMF - Guelph - Simon Frasier UTK - ORAU - ORNL Reactor Science LSU - Mississippi - Warsaw Verification the absolute βn branching ratios I βn for N=48 79 Ga, N=49 80 Ga, N=50 81 Ga, and measure for the first time I βn value for N=51 83 Ge. Proposed experiment represents a step towards the understanding and improving of ISAC discovery potential for decay studies of new nuclei produced in fission. (before the high resolution mass separator will be operating at TRIUMF). 3Hen at TRIUMF K. P. Rykaczewski

230 ms 480 ms HRIBF β-  -n-TAS-E n results marked in red βn branching ratios around N=50, for 79 Ga, 80 Ga, 81 Ga and 83 Ge precursors, to be measured at TRIUMF-ISAC 484 ms 221 ms

Decay studies with post-accelerated fission products Range out experiment +/-40 keV +/-160 keV HRIBF ~ 0.5 % overall efficiency Mass separator M/ΔM ~ MeV protons  A 2-3 MeV/u HRIBF charge exchange cell ~ 5% efficiency Positive ions Tandem accelerator (negative ions only) ~ 10% efficiency ISAC ~ 0.1 % overall efficiency IRIS-1 ORIC : 238 U fission fragments ~10 11 /s gas cell beam kicker Energy loss Total ion energy 76 Cu 76 Ga 76 Ge no 76 Zn !!! Range out exp gas cell spectra C.J.Gross et al., EPJ A25,115,2005 TRIUMF 500 MeV ~ 10  A 6 g 25 g   ~70%  n ~44%   ~4% Laser ion source at TRIUMF Isobar separator M/  M ~ 10000

Why the decay spectroscopy of fission products is difficult at ISAC ? 1.beams of interesting ions come with usually much stronger contamination of higher-Z fission products (this problem is not ISAC specific). But here the selective laser ion source combined with ranging-out technique can help to purify Ga and Ge ion beams. 2.ISAC post-acceleration scheme requiring a Charge State Booster creates a lot of stable ion contamination in the post-accelerated beams (see proposal). But these are stable contaminants – no radiation emitted ! Higher-Z contaminants should be stopped in our Ionization Chamber. Unfortunately, the combination of selective ionization, ranging-out and radiation detection is not (yet) an universal solution to the beam contamination problem – every effort to characterize and reduce the beam contamination at TRIUMF is extremely important But this experiment can help to advance beam purification techniques at ISAC

3He tubes Ranging-out of higher-Z components, counting and transmitting Ga ions, deposition on Ed Zganjar’s MTC followed by a transport and β-n-  detection This experimental setup is able to remove the radioactive isobaric contaminants. However, stable ions can overload the ionization chamber (tested up to 300 kHz ion rate) making the event-by-event identification and detection difficult to impossible. The proposed solution: we reduce the beam intensity to the limit of, e.g., 81 Ga detection in the IC and βn  setup (factor 1000). We measure the absolute rate of 81 Ga ions vs the β  rate and optimize the gas pressure. Later we use the IC in a passive degrader mode and obtain the rate of implanted 81 Ga ions from β  counts. MCP can count reliably up to 10 6 pps, but it counts full ion beam intensity before IC.

300 kHz 250 kHz 225 kHz 150 kHz 76 Ge 76 Se 76 Ge 76 Se 76 Ge 76 Se 76 Ge 76 Se The advantages of a small transmission ion chamber Short cathode to anode distance (~3 cm) drift time minimized over 1.5 cm of beam-anode distance CF 4 gas a fast gas (drift time ~100 ns/cm) Total path length in gas (~7 cm) compact to minimize beam blow up Segmented anode (6 electrically separated) measured energy loss dependent on total energy a measure of range (cm scale) different combinations of anodes can select and isolate events of interest Small windows with metal support wires mylar windows of 0.9 μm and 2.4 μm 0.9 μm withstands excess of 200 Torr modular design easy to replace window quickly openings 16 mm diameter Recent addition (untested) central wire “guard ring” to smooth electric field gradient near window to improve anode 1 & 6 performance

Background current observed from the CSB during 2011 development runs. The vertical lines show the transmitted region that corresponds to the A/q for 79 Ga 12+. The 46 Ti 7+, 79 Br 12+, 86 Kr 12+, 92 Mo 14+, 131 Xe 20+, and 132 Xe 20+ ions are responsible for this current. Worst case scenario: 100 pA of 46 Ti 7+ means ions/s. With factor 10 3 down (slits closed), we have pps in IC. 79 Ga +12 Projected acceptance of the ISAC-I RFQ. The region labeled “B” maximizes A=79 transmission, and reduces possible 86 Kr and 131 Xe by an order of magnitude. Fine-tuning of the phase of the RFQ, depending on the exact stable-beam composition, can significantly reduce the background. 79

The proposed experiment represents an ideal commissioning experiment for the Charge State Booster, especially when considering that we can give instantaneous online feedback of the beam composition, and provided that the stable-beam rates are not in excess of ~ 10 9 pps the measurement can be successful. The presented project has an obvious continuation path towards new exotic beta-delayed neutron emitters, like 86 Ga and even 87 Ga, after demonstrating successful study of activities produced at higher rates. (let’s keep 3Hen at TRIUMF, if we can get new results there)

K. Rykaczewski, TRIUMF, 12th July 2012 Beam time accepted: I βn for 79 Ga, 80 Ga and 81 Ga, for each ion: one 8-hour shift of tuning plus one 8-hour shift for measurement amount to four 12-hours shifts 85 Ga rate (towards new nuclei): two 12-hour shifts I βn for 83 Ge: one 8-hour shift tuning plus six 8-hour shifts for measurement amount to about five 12-hour shifts Total of eleven 12-hour shifts

Detectors for beta decay studies 3Hen array after “ranging-out” hybrid 3Hen-β-  array at LeRIBSS ε n ~80% ε n ~44% K. Rykaczewski, TRIUMF, 12 th July 2012 Ed Zganjar mounting 3Hen array at LeRIBSS